Create a discretional string(C#)

Here is a interview exam, I write it with RanDom class. OK Let's go!!

namespace StringBuildTest
{
class StringBuildT
{
string[] str = new string[] { "a", "b", "c", "d","e","f","g","h","i","j","k","l","m","n",
"o","p","q","r","s","t","u","v","w","x","y","z"};
StringBuilder SB = new StringBuilder();
Random rd = new Random();
string strOut = "";//length is 1000
public void DsiplayString()
{
for (int i = 0; i < 1000; i++)
{
SB.Append(str[(int)rd.Next(26)].ToString());
}
strOut = SB.ToString();
Console.WriteLine(strOut);
}
}

class Program
{
static void Main(string[] args)
{
StringBuildT sbt = new StringBuildT();
sbt.DsiplayString();
}
}
}